From: crash!bblue@SDCSVAX.ARPA
Date: Sun, 23 Jun 85 22:28:22 PDT
To: info-apple@brl
Subject: Christensen Protocol extensions for Apple

In response to several requests regarding the extensions to the Christensen
Protocol that are implemented in our ASCII Express "Pro" series of software:

--------------------------
		      Ascii Express "The Professional"
		   Extensions to the Christensen Protocol
		     This document Copyright (c) 1985 by
			 Mark Robbins and Bill Blue,
		      United Software Industries, Inc.
			   Document date: 06/02/85

 
Ascii Express "The Professional" (ProDOS and DOS 3.3 versions) hereafter
referred to as AEPro, uses Christensen non-batch checksum protocol, with one
(DOS 3.3) or two (Pro-DOS) extensions for the Apple world.  Throughout this
discussion, it is assumed you have a working knowledge of the original
Christensen protocol.

There is one additional packet at startup exchanged when transferring between
two DOS 3.3 AEPro's, and two additional packets exchanged (one at startup and
one at the end) between two ProDOS AEPro's.  A STARTUP PACKET tells the
receiving side that a DOS 3.3 file type is about to be sent, and identifies
that type.  It also indicates to a ProDOS AEPRO that a FINAL PACKET
will be sent at the end of the DATA PACKETS.  Prior to receipt of the STARTUP
PACKET the receiving AEPro will have already opened the specified file as a
standard ($00 filetype) textfile.  After the STARTUP PACKET is received and
the filetype is not $00, the file is deleted and reopened under the correct
filetype.  If no Special Start Of Header is received, AEPro will fall into
standard Christensen receive, leaving the filetype as Text.  The format of
the STARTUP PACKET is:

Byte 1         Special Start Of Header  SOH+msb  ($81)
 
Byte 2         DOS 3.3 file type (MSB stripped)   See "Beneath Apple Dos"
	       AEPro ProDOS version always sends $00
 
Byte 3         Complement of byte 2               (xor Byte 2 with $FF)
 
The receiving side must send an ACK ($06) to the sending side to acknowledge
successful reception of the STARTUP PACKET.  Successful reception is assumed
if bytes two and three are received as complements of each other.

The regular Christensen protocol begins for all remaining DATA PACKETS.  If
the receiver is not an AEPro, but is some other implementation of Christensen
protocol, it will NAK ($15) the Special Start Of Header.  Upon receipt of a
NAK, the sender will try two more times, and then fall into standard protocol
mode.

The FINAL PACKET will be received only if the file is being sent by a ProDOS
AEPro.  This packet is designed to tell the receiving side all about the
file; i.e., filetype, number of blocks used, the modification date and time,
End of File marker, etc.

At the conclusion of all the DATA PACKETS, receipt of the final EOT
($04) from the sender, responding to it with an ACK, and if a STARTUP
PACKET was received at the beginning of the exchange, the ProDOS receiving
side then sends a SYN ($16) and waits two seconds for an ETB ($17) from the
sender. It retries the SYN and waits for an ETB up to three times before
giving up. If the ETB is received then it sends a NAK to start the FINAL
PACKET.

If all went well to this point, the receiver expects a block of data in the
following format.  The block is identified as number $AA.

	{$01} {$AA} {$55} {128 data bytes} {checksum of the data bytes}

This packet is handled the same way as DATA PACKETS, ACK if okay and NAK if
not.  Checksums are calculated by adding all data bytes mod 256, with any
carry ignored.  Currently, of the 128 bytes of data, only the first 23 bytes
are currently significant.  Essentially, these bytes are made up of the
ProDOS GET_FILE_INFO and GET_EOF paramater lists.  For the exact meaning of
each byte, refer to the Apple ProDOS Technical Reference Manual.

GET_FILE_INFO Parameter List:

Byte 1         param_count (always send $0A)
Byte 2         fixed_param (always send $D4)
Byte 3         fixed_param (always send $41)
Byte 4         access
byte 5         file_type
Byte 6         low aux_type
Byte 7         high aux_type
Byte 8         storage_type
Byte 9         low blocks_used
Byte 10        high blocks_used
Byte 11        byte 0 mod_date
Byte 12        byte 1 mod_date
Byte 13        byte 0 mod_time
Byte 14        byte 1 mod_time
Byte 15        byte 0 create_date
Byte 16        byte 1 create_date
Byte 17        byte 0 create_time
Byte 18        byte 1 create_time

SET_EOF Parameter List:

Byte 19        param_count (always send $02)
Byte 20        ref_num (doesn't matter)
Byte 21        EOF Low byte
Byte 22        EOF Middle byte
Byte 23        EOF High byte


--------------------------

--Bill Blue	crash!bblue@ucsd
		{ihnp4, cbsogd, sdcsvax, noscvax}!crash!bblue